Since rich scatters often exist on the ground, as in [1]}, we adopt the Rayleigh fading to model the direct channel as
\( \mathbf {d}_k = \sqrt{\gamma _k} \tilde{\mathbf {d}}_k \) ,
where \(\gamma _k\)  denotes the path-loss.
The entries of \(\tilde{\mathbf {d}}_k\)  are independent and identically distributed (i.i.d.) complex Gaussian random variables, and we have \( \tilde{\mathbf {d}}_k \sim \mathcal {CN}\left({\bf 0},\mathbf {I}_M\right)\) .
